The present invention relates to a device for detecting a displacement between patterns particularly adapted for use in a scanning system of an optical character reader.
There have been devised and demonstrated various scanning systems for optical character readers. In one system a mechanical scanner or flying-spot scanner including a point source of light is used for two-dimensionally scanning a pattern bearing sheet so that light reflected back from the sheet may be converted into electrical signals by a receptor, such as a photoelectron multiplier. In another system a character or pattern bearing sheet is one- or two-dimensionally illuminated by a light source, such as a lamp, and a one-dimensional receptor, such as a photocell, scans the sheet in a one-dimensional manner. In a further system, a pattern bearing sheet is two-dimensionally illuminated and a two-dimensional image-pickup device is used to two-dimensionally pick up patterns or characters. However, the above-mentioned prior art scanning systems have some problems to be described below. First, in the system wherein the one-dimensional scanner scans patterns or characters one-dimensionally, the linearity of the scanned pattern is good in the direction in which the scanning elements are arrayed, but the image scanned pattern is distored in the scanning direction. In the system wherein the two-dimensional scanner is used, a pattern without accompanying scanning distortion may be obtained, but when the scanner cannot cover a pattern field of a sufficiently large area, two-dimensional patterns must be correctly connected to each other. Recently, two-dimensional photocells have been devised and demonstrated, so that scanners are made very compact in size to such an extent that a manual scanner which may be held by an operator for manual scanning may be provided. With the manual scanner the connection or continuity between the pattern images presents a very important problem.
To solve this problem, there has been devised and demonstrated a system in which every time an individual pattern in the receptor field is picked up, any overlapped portions are eliminated from the read-out result. However, the use of this system is limited to an optical character reader having a sufficiently high processing speed.
The manual scanners have a defect that unless correctly operated an optical character reader cannot correctly sense and recognize. For instance, if an operator fails to scan scan in parallel with a row, the detected character will be skewed. If the scanning speed is too fast, the next character is picked up before the preceding character has been processed and recognized. On the other hand, when the scanning speed is too slow, the same character may be picked up twice. One of the methods for solving these problems is to give suitable information to an operator so that the correct manual scanning may be accomplished.